I agree that you should try to get a fresh key made first.
However, the lock tumblers DO wear, although slightly. I have a good locksmith that I've been going to for several years and he'* made me brand new keys from the old ones. Places like wal-mart really won't be able to.... they're only trained to run a machine, not understand how to make a key from scratch. Also, my locksmith usually lightly buffs the edge of the key so its not quite as "fresh" to match the the wear on the tumblers.
If the key doesn't work, then its time to replace the cylinder. I had one go bad on me for some unknown reason and to get it to turn I had to place the key in the ignition and use a big pair of vice-grips to get it turn. It worked fine
After you get it somewhere you can work on it you'll need to go to an Autozone and rent the following tools: lock plate spring compressor, steering wheel puller.
...and you're going to need a set of sockets and either small screwdrivers or picks.
First, remove the horn button. These just pull off, but sometimes they need a bit of leverage from a screwdriver.
Next, Remove the odd retaining clip over the wheel retaining nut. Use a screwdriver or pick. Set aside.
Remove the wheel retaining nut. I *think* its 19mm, so the factory lug wrench might work. Otherwise, you need a socket.
At this point the wheel is free, but to get it to pop off the splines of the column, you need to use the steering wheel puller. Use two bolts (should be supplied with the kit) to screw down two sides of the puller into the wheel (should have some threaded holes) and then turn the center stud clockwise against the column (where you removed the retaining nut)
Now the steering wheel should be free. At this point, you need to use the lock plate compressor to push it down enough that you can easily access the retaining ring. This ring sits in a recess in the column that keeps the plate in place from pressure put on it by the underlying spring. You have to get that stupid retaining spring loose. ...use small screwdrivers or picks...anything. This is the hardest part.
After that is removed, you should be able to see the white plastic turn signal cam and maybe a torx screw situated above where the lock cylinder is. If you can get at the torx screw without removing the turn signal cam then do so but I don't remember if you need to remove the cam (its like 3 phillips screws tho, so its easy).
Got the torx nut out? A screwdriver will work fine, its not torqued down real tough. Now just slide out the old cylinder. Thats it. One little screw holds that thing in.
....in conclusion, hope that its just the key